1. Field of the Invention
This invention relates to the manufacture of semiconductor substrates such as wafers and, more particularly, to a method for monitoring the state of polarization incident on a photomask in projection printing using a specially designed polarization monitoring reticle for high numerical aperture lithographic scanners.
2. Description of Related Art
As is well known, in a typical photolithographic process, a thin layer of a photosensitive material or photoresist is deposited over a semiconductor layer on a wafer. Each wafer typically is used to make many chips. During the photolithography process, illumination such as ultra-violet light is illuminated through a lens system and a photolithographic mask or reticle to a chip area on the semiconductor wafer. The reticle has a particular desired device or circuit pattern and the pattern is exposed over a portion of the chip area by the illumination to create exposed and unexposed regions on the chip. These exposed or unexposed regions are then removed and the underlying layer is etched to create the desired pattern in the semiconductor layer. The etched semiconductor layer is then further processed to create the desired circuit or device portion in that layer. This photolithography process is repeated multiple times on successive layers of the wafer to define many circuit elements on the chips on the wafer. At the end of the photolithography process, the wafer is cut into the completed semiconductor chips.
Typically, a reticle is made from a transparent plate and has a device exposure region and an opaque region. The plate is often made of glass, quartz, or the like and the opaque chrome region typically includes a layer of chrome. The device exposure region generally has a square or rectangular shape and is positioned in the center of the reticle. The device exposure region includes transparent portions and opaque portions defining a device pattern. The transparent portions in the device exposure region allow illumination from a light source to travel through them and reach the wafer. On the other hand, the opaque regions of the device region block the light and the light does not reach the wafer.
Manufacturers of microelectronic circuits are continually seeking to produce features having smaller dimensions. The lithographic production of such features typically uses a step-and-scan imaging tool 120, as shown in FIG. 1, to project a pattern onto a photosensitive resist layer on a substrate or wafer. The projection optical system of the imaging tool includes a lamp, laser, or other optical source 122 that projects radiation 124 used to illuminate a photomask or reticle 128 through a condenser lens system 126. The photomask or reticle 128 contains the pattern to be projected and reproduced on the wafer substrate, and is generally oriented substantially perpendicular to an optical axis 124 of the projection optical system. Some of the light radiation 146 that passes through the photomask 128 is collected by the projection optics 134 and the aerial image 136 of the pattern produced by passage of radiation 146 through the mask is directed onto the wafer 142, so as to create the pattern or image 140 on the wafer.
In a step-and-scan system, the photomask 128 and the wafer 142 are mounted on mask stage 133 and substrate stage 138, respectively, that move relative to the fixed optical system. The optical system contains an aperture or slit 132 through which light is allowed to pass to the reticle. The entire mask pattern within the desired transfer region of a reticle is completely exposed by scanning along the one-dimensional scan direction 130 and across the complete one-dimensional width of the transfer region to produce a complete pattern 140 on the wafer resist, for example a complete chip pattern. The scanning process is subsequently repeated to produce the desired number of patterns on the wafer 142.
In order to produce features having smaller dimensions in the manufacture of microelectronic circuits, three factors, a phenomenological process resolution factor (k1), the light wavelength (λ) and the numerical aperture value (NA) are involved in the lithographic processing that may be used to create the minimum line width (Wmin) according to a standard generalization of Rayleigh's equation:Wmin=k1λ/NA 
Sometimes a slightly different value of k1 is used that relates λ and NA to the half-pitch of a periodic system of lines and spaces.
To enable use of finer features in integrated circuits many advances have been made in lithographic technology that allow smaller values of k1. In the early days of integrated circuit manufacture only k1 values above 1 were practical, but now k1 values near 0.3 are being employed, and further reductions are sought. A difficulty here is that image contrast is degraded at such low k1 values, making it difficult to achieve size uniformity in the printed circuit features as distributed over the chip, such size uniformity usually being required for acceptable circuit performance.
Developments of new tools and methods in lithography have led to improvements in resolution of the imaged features patterned on a device such as a wafer possibly leading to a resolution of less than 50 nm. This may be accomplished using relatively high numerical aperture (NA) lenses, wave lengths down to 157 nm and a plethora of techniques such a phase shift masks, non-conventional illumination and advanced photoresist processing. Looking at the NA (numerical aperture) factor, recent advances have enabled exposure tool manufacturers to make tools with NA values in excess of 0.70, 0.75, 0.80 and higher, and tools with NA values of 0.93 are now available. NA values higher than 1.0 are also currently achieved with the use of immersion imaging, where ultra pure water is placed between the last lens element and the photoresist. Future use of liquids with a higher index of refraction may enable NA values higher than 1.35; perhaps up to around 1.8. Because modern exposure tools have such high NA values, images must be formed using waves with high angles of propagation within the resist, i.e., large propagation angles with respect to a direction normal to the surface of the resist layer.
At the high numerical apertures that produce such incidence angles, it has been observed that there is a fundamental loss of image contrast for the transverse magnetic (TM) polarization of the light waves. Even when the source radiation is engineered to be transverse electric (TE) polarized to improve contrast of a particular feature orientation, a small amount of TM light is likely to be present. Furthermore, the ratio of TE to TM light is likely to vary across the imaging field. Depending on the NA, feature type and size, illumination configuration, and other imaging characteristics, this variation may cause distortions of the desired image across the imaging field.
With high NA projection systems it is desirable to provided polarized or at least partially polarized illumination radiation. This enables image formation at a wafer level to be enhanced by using radiation with a state of polarization which is either best suited for imaging of a particular feature, or a best case compromise among various feature types and sizes. The desired polarization characteristics of the illumination system are typically expressed as a set of Stokes parameters for each illumination pupil-fill location. The Stokes parameters are a mathematical means to completely describe the time-averaged polarization characteristics of a ray of light. This desired polarization-dependent pupil-fill function describes the intended polarization characteristics of each ray within the cone of light that illuminates the mask. The pre-determined target polarization-pupil-fill can be stored as data in a memory device and compared with the actual polarization radiation existing in the scanning process.
In U.S. Pat. No. 7,224,458 one of the inventors herein developed a method to monitor the state of polarization incident on a photomask in projection printing. The method includes a series of phase-shifting mask patterns to take advantage of high NA effects to create a signal depending on only one incident polarization component. A test reticle design is shown consisting of multiple polarimeters with an array of pin holes on the back side of the photomask. This technique is able to monitor any arbitrary illumination scheme for a particular imaging tool. The reticle comprises a set of up to six calibrated PSM analyzers for each pupil-fill coordinate for each field position of interest. These analyzers, also referred to as monitors, enable the calculation of the Stokes parameters and completely characterize the state of incident polarization in the high NA projection printing systems. In polarimetry, a set of analyzers are used to each measure the flux (F), one polarization component in the incident light. To account for the fact that the analyzer does not consist of the perfect polarization elements, the analyzers are first calibrated producing a polar unit metric measurement matrix (W) for each set of up to six analyzers. This calibration data has been used to determine the measured Stokes parameters describing the polarization states from any arbitrary illumination by solving a set of linear equations.
A proposed reticle design is shown in FIG. 1 of the patent showing the front and back side of the reticle for a particular field location. Multiple sets of polarimeters are used in a cluster near each pin hole location where each polarimeter set has up to six analyzers and a unique period and orientation of the four-phase linear progression depending on its relative locations of the pin holes, or likewise, the desired σ measurement.
FIG. 2 shows a monitor similar to the prior art existing monitor as in U.S. Pat. No. 7,224,458. This monitor gives a signal for TE and TM polarization and their bisectors (45°, 135°), and uses a four phase grating 69 that is always oriented parallel to the axis at a given location within the cluster.
There still exists, however, a need to monitor polarization in high numerical aperture lithographic scanners and the subject invention is an improvement of the test reticle and technology disclosed in U.S. Pat. No. 7,224,458, the disclosure of which is hereby incorporated by reference.